The present invention relates to the use of Code Division Multiple Access (CDMA) communications techniques in cellular radio telephone communication systems, and more particularly, to a method and system related to handover of connections between frequencies using non-continuous Direct Sequence-Code Division Multiple Access (DS-CDMA) transmissions.
DS-CDMA is on type of spread spectrum communications. Spread spectrum communications have been in existence since the days of World War II. Early applications were predominantly military oriented. However, today there has been an increasing interest in using spread spectrum systems in commercial applications. Some examples include digital cellular radio, land mobile radio, satellite systems and indoor and outdoor personal communication networks referred to herein collectively as cellular systems.
Currently, channel access in cellular systems is achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In FDMA, a communication channel is a single radio frequency band into which a signal's transmission power is concentrated. Interference with adjacent channels is limited by the use of band pass filters which pass substantial signal energy only within the specified frequency band. Thus, with each channel being assigned a different frequency band, system capacity is limited by the number of available frequency bands as well as by limitations imposed by frequency reuse.
In TDMA systems which do not employ frequency hopping, a channel consists of a time slot in a periodic train of time intervals over the same frequency band. Each period of time slots is called a frame. A given signal's energy is confined to one of these time slots. Adjacent channel interference is limited by the use of a time gate or other synchronization element that passes signal energy received at the proper time. Thus, the problem of interference from different relative signal strength levels is reduced.
With FDMA or TDMA systems (or hybrid FDMA/TDMA systems), one goal is to insure that two potentially interfering signals do not occupy the same frequency band at the same time. In contrast, Code Division Multiple Access (CDMA) is an access technique which uses spread spectrum modulation to allow signals to overlap in both time and frequency. There are a number of potential advantages associated with CDMA communication techniques. The capacity limits of CDMA-based cellular systems are projected to be higher than that of existing analog technology as a result of the properties of wideband CDMA systems, such as improved interference diversity and voice activity gating.
In a CDMA system the data stream to be transmitted (i.e., a symbol stream which has undergone channel encoding etc.) is impressed upon a much higher rate data stream known as a signature sequence. Typically, the signature sequence data (commonly referred to as "chips") are binary or quaternary, providing a chip stream which is generated at a rate which is commonly referred to as the "chip rate". One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The symbol stream and the signature sequence stream are combined by multiplying the two streams together, assuming the binary values of the two streams are represented by +1 or -1. This combination of the signature sequence stream with the symbol stream is called spreading the symbol stream signal. Each symbol stream or channel is typically allocated a unique spreading code. The ratio between the chip rate and the symbol rate is called the spreading ratio.
A plurality of spread signals modulate a radio frequency carrier, for example by quadrature phase shift keying (QPSK), and are jointly received as a composite signal at a receiver. Each of the spread signals overlaps all of the other spread signals, as well as noise-related signals, in both frequency and time. If the receiver is authorized, then the composite signal is correlated with one of the unique codes, and the corresponding signal can be isolated and decoded.
For future cellular systems, the use of hierarchical cell structures will prove valuable in even further increasing system capacity. In hierarchical cell structures, smaller cells or micro cells exist within a larger cell or macro cell. For instance, micro cell base stations can be placed at a lamp post level along urban streets to handle the increased traffic level in congested areas. Each micro cell might cover several blocks of a street or a tunnel, for instance while a macro cell might cover a 3-5 Km radius. Even in CDMA systems, the different types of cells (macro and micro) will operate at different frequencies so as to increase the capacity of the overall system. See, H. Eriksson et al., "Multiple Access Options For Cellular Based Personal Comm.," Proc. 43rd Vehic. Tech. Soc. Conf., Secaucus, 1993. Reliable handover procedures must be supported between the different cell types, and thus between different frequencies so that mobile stations which move between cells will have continued support of their connections.
There are several conventional techniques for determining which new frequency and cell should be selected among plural handover candidates. For example, the mobile station can aid in the determination of the best handover candidate (and associated new base station) to which communications are to be transferred. This process, typically referred to as mobile assisted handover (MAHO), involves the mobile station periodically (or on demand) making measurements on each of several candidate frequencies to help determine a best handover candidate based on some predetermined selection criteria (e.g., strongest received RSSI, best BER, etc.). In TDMA systems, for example, the mobile station can be directed to scan a list of candidate frequencies during idle time slot(s), so that the system can determine a reliable handover candidate if the signal quality on its current link degrades beneath a predetermined quality threshold.
In conventional CDMA systems, however, the mobile station is continuously occupied with receiving information from the network. In fact, CDMA mobile stations normally continuously receive and transmit in both uplink and downlink directions. Unlike TDMA, there are no idle time slots available to switch to other carrier frequencies, which creates a problem when considering how to determine whether handover to a given base station on a given frequency is appropriate at a particular instant. Since the mobile station cannot provide any inter-frequency measurements to a handover evaluation algorithm operating either in the network or the mobile station, the handover decision will be made without full knowledge of the interference situation experienced by the mobile station, and therefore can be unreliable.
One possible solution to this problem is the provision of an additional receiver in the mobile unit which can be used to take measurements on candidate frequencies. Another possibility is to use a wideband receiver which is capable of simultaneously receiving and demodulating several carrier frequencies. However, these solutions add complexity and expense to the mobile unit.
In the parent patent application to Willars et al., this problem is addressed by introducing discontinuous transmission into CDMA communications techniques. For example, a compressed transmission mode is provided using a lower spreading ratio (i.e., by decreasing the number of chips per symbol) such that with a fixed chip rate the spread information only fills a part of a frame. This leaves part of each frame, referred to therein as an idle part, during which the receiver can perform other functions, such as the evaluation of candidate cells at other frequencies for purposes of handover.
This solution is readily applicable to CDMA systems wherein non-orthogonal code words are used to spread the information data sequence. In these types of systems, commonly referred to as "long code" systems, one signature sequence is much longer than one symbol (often billions of symbols long). Since these codes are non-orthogonal to begin with, temporarily changing the spreading ratio of one or several channels to provide compressed mode transmissions does not create extra inter-code interference.
The solution proposed in the parent application becomes problematic, however, for DS-CDMA systems where orthogonal code words are used to spread data streams. In so-called "short" code systems, a short code set (e.g., including 128 codes of length 128 chips) is chosen so that all codes are orthogonal to each other over one symbol interval, i.e., over the length of the code. If the spreading factor is altered for a transmissions in the downlink to a user in this type of system, that user's code would no longer be orthogonal to the other users over a symbol interval. This, in turn, would create potentially undesirable inter-channel interference.
Accordingly, it would be desirable to provide a DS-CDMA system in which transmission and reception was discontinuous but which did not rely on a reduction in the spreading ratio to provide idle time for the receiver to measure on different frequencies.